Press and hold the power button for 10 seconds, then press it again. If that doesn't help, repeat after unplugging accessories from your Mac, including printers, drives, USB hubs, and mobile devices. (You could have an issue with one or more of those devices or their cables.)

If nothing appears on your display, make sure that its brightness is turned up. Mac notebook computers have a Brightness Up key on the built-in keyboard. If you're using an external display, make sure that it's turned on and isn't experiencing any other issues.